1. Field of the Invention
This invention relates to a pattern for determining the optimum position of an object with respect to the focal point of a beam of radiation focused on the object. In particular, it relates to a reticle pattern formed on a reticle for determining the optimum position of a wafer with respect to the focal point of a beam of light which projects an image on the surface of the wafer as part of a semiconductor manufacturing process. Moreover, it relates to a method for estimating the optimum position of a wafer or other object with respect to the focal point of a beam of light by the use of a reticle pattern formed on a reticle.
2. Description of the Related Art
A conventional reduction projection exposure method for forming high resolution image patterns on a wafer by the use of a mask is described in "VLSI Technology Guide Nyumon!" (Heibonshia), p. 143-144. An advantage of this method is that the mask is not damaged during the exposure process because the mask and wafer do not come into contact with each other thereby ensuring that the mask remains free of defects that might result from such contact. Another advantage of the process is that it results in a relatively high yield of the semiconductor product being manufactured. Apparatuses used for the reduction projection exposure method are referred to as steppers, an example of a conventional stepper being shown in FIG. 17.
In FIG. 17, a mercury vapor lamp 171 provides a source of light that is passed through a condenser lens 172 and a filter (not shown) along a vertical Z-axis to provide a beam of substantially single frequency light. The filtered light beam impinges on a reticle 173 having deposited thereon an original of an image pattern consisting of opaque and transparent areas, the letter "A" being shown in FIG. 17 as an example. The light passes through the transparent portions of the image pattern and is focused, after passing through a reduction lens 174, onto the surface of a wafer 175 covered with a photoresist material. The light is automatically focused on the wafer, which is mounted on a stage 176 movable along X, Y and Z axes, under predetermined exposure conditions by means of a sensor (not shown) located near the light source. The sensor detects the relative position of the photoresist covered surface of the wafer with respect to the focal point of the focused light. Means (not shown) are provided for changing the relative positions of the wafer and the focal point of the focused light along the Z-axis.
A disadvantage of this system in that the relative position of the wafer along the Z-axis, as set by the sensor, and the focal point at which the optical system brings the light into focus are not always matched. Consequently, any deviation in the position of the wafer surface from the optimum position must be measured and corrected.